Method for controlling the ink feed of a printing machine for half-tone printing

Abstract

A method for controlling the ink feed of a printing machine, especially an offset printing machine, for printing four-color half-tone combined prints. In selected test regions in an original and in the corresponding test regions in the printed sheet, the standard color values of the four-color overprint are determined. The infrared color density for the black printing ink is determined in the near infrared. The standard color values of the four-color overprint are converted by means of a linear transformation into standard color values corresponding to the combined print of only the three chromatic colors. The coefficients of the linear transformation are determined empirically as a function of the infrared color density of the black printing ink. From the standard color values of the three-color print, the effective surface coverage values of the three chromatic colors are determined via the modified Neugebauer equations. The effective surface coverage value of the black ink is determined from the infrared color density according to an empirical relationship. The settings for the ink feed elements of the printing machine are adjusted according to the differences in the effective surface coverage values between the original and the printed product.

What is claimed is: 
     
       1. A method for controlling the ink feed elements of a printing machine operated in half-tone, in particular an offset printing machine, the method comprising the steps of: selecting test regions on an original and corresponding test regions on a printed product printed in printing inks of three chromatic colors and the color black;   detecting photoelectrically reflectance values of the selected test regions, wherein for the black printing ink the reflectance is detected in the near infrared spectral range;   determining the infrared color density for the black printing ink from the reflectance in the near infrared spectral range;   determining from the reflectance values of the test regions the standard color values of the four-color printing;   converting via a linear transformation the standard color values of the four-color printing to the standard color values of a three-color printing corresponding to a color locus which is produced by the combined print of only the three chromatic color, the coefficients of the linear transformation being determined empirically as a function of the infrared color density of the black ink;   determining the optically effective surface coverage values of the three chromatic colors from the standard color values of the three-color printing;   determining the optically effective surface coverage value of the black ink from the infrared color density via an empirically determined relationship;   adjusting the settings for ink feed elements of the printing machine according to the differences in the effective surface coverage values between the original and the printed product.   
     
     
       2. A method as in claim 1, wherein the effective surface coverage values of the three chromatic colors are determined from the standard color values of the three-color printing using the Neugebauer equations, and the standard color values being used in the Neugebauer equations for the individual color combinations and the overprinting combinations are determined on half-tone areas in printing trials. 
     
     
       3. A method as in claim 1, wherein the method is repeated until the color deviations lie within a predetermined tolerance structure. 
     
     
       4. A method as in claim 1, wherein the method includes the further step of determining empirically, as a function of the infrared color density of the black ink, the coefficients of a linear transformation which converts the standard color values of a four-color printing into the standard color values of a corresponding three-color printing.